The present invention relates to a manufacturing method for a connection between a valve head and a valve stem of a multi-part valve, to a connection made using the method, and to an automotive engine valve made using the method.
Valves used in mass production are mostly based on high-temperature resistant steel, at least in the valve head area. The valve stem is made of less highly alloyed steel and is connected to the valve head by friction welding. In the valve seat area, valve heads are either plasma-coated with a wear-resistant material or hardened. From racing, valves are known that are made of titanium and TiAl. Currently, it is being considered to manufacture and use powder-metallurgically produced or cast solid valves of TiAl.
However, cast solid valves can be manufactured using by centrifugal casting or using a kind of a pressure diecasting or injection method. In order to avoid pores in the stem area which is difficult to feed, appropriate preheating must be provided in the permanent molds used. For this purpose, correspondingly expensive permanent mold materials are needed, such as niobium or tantalum. Moreover, the preheating operations increase cycle the times during production. Heretofore, however, it has not yet been possible to avoid residual porosity in the stem, even under optimum temperature control.
In the case of multi-part valves, different requirements are placed on the head and the stem. In particular, valve heads must be highly resistant to temperature and wear, whereas the valve stem must have a high strength in conjunction with a high resistance to abrasion at the stem end. The most convenient material is chosen in each case according to the requirements placed on the valve parts. When using valve heads, for example, of TiAl, the stem can be chosen to be made of suitable steel.
Conventional approaches to produce multi-part valves are limited to manufacturing the valve head and the stem separately from each other and to interconnect them in a subsequent process step.
U.S. Pat. No. 4,834,036 describes a method for making an interconnection between a valve head and a valve stem which is hollow inside. During manufacture, the stem end which is inserted in the head is expanded and connected thereto in a positive-locking manner under the influence of heat with the aid of a pressure medium which is pressed into the hollow valve stem.
Apart from single-part models, multi-part valves have the disadvantage of having to ensure a suitable connection of the individual parts.